Add some metric history

The count of connections and count of delivered messages now have 50
minutes of history available in the /about sparklines.

smtpd/listener.go

@@ -1,11 +1,13 @@
 package smtpd
 
 import (
+	"container/list"
 	"expvar"
 	"fmt"
 	"github.com/jhillyerd/inbucket/config"
 	"github.com/jhillyerd/inbucket/log"
 	"net"
+	"time"
 )
 
 // Real server code starts here
@@ -19,7 +21,12 @@ type Server struct {
 
 var expConnectsTotal = new(expvar.Int)
 var expConnectsCurrent = new(expvar.Int)
+var expConnectsHist = new(expvar.String)
 var expDeliveredTotal = new(expvar.Int)
+var expDeliveredHist = new(expvar.String)
+
+var deliveredHist = list.New()
+var connectsHist = list.New()
 
 // Init a new Server object
 func New() *Server {
@@ -60,9 +67,34 @@ func (s *Server) Start() {
 	}
 }
 
+// When the provided Ticker ticks, we update our metrics history
+func metricsTicker(t *time.Ticker) {
+	ok := true
+	for ok {
+		_, ok = <-t.C
+		expDeliveredHist.Set(pushMetric(deliveredHist, expDeliveredTotal))
+		expConnectsHist.Set(pushMetric(connectsHist, expConnectsTotal))
+	}
+}
+
+// pushMetric adds the metric to the end of the list and returns a comma
+// separated string of the previous 50 entries
+func pushMetric(history *list.List, ev expvar.Var) string {
+	history.PushBack(ev.String())
+	if history.Len() > 50 {
+		history.Remove(history.Front())
+	}
+	return JoinStringList(history)
+}
+
 func init() {
 	m := expvar.NewMap("smtp")
 	m.Set("connectsTotal", expConnectsTotal)
+	m.Set("connectsHist", expConnectsHist)
 	m.Set("connectsCurrent", expConnectsCurrent)
 	m.Set("deliveredTotal", expDeliveredTotal)
+	m.Set("deliveredHist", expDeliveredHist)
+
+	t := time.NewTicker(time.Minute)
+	go metricsTicker(t)
 }

smtpd/utils.go

@@ -1,6 +1,7 @@
 package smtpd
 
 import (
+	"container/list"
 	"crypto/sha1"
 	"fmt"
 	"io"
@@ -25,3 +26,15 @@ func HashMailboxName(mailbox string) string {
 	io.WriteString(h, mailbox)
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
+
+// JoinStringList joins a List containing strings by commas
+func JoinStringList(listOfStrings *list.List) string {
+	if listOfStrings.Len() == 0 {
+		return ""
+	}
+	s := make([]string, 0, listOfStrings.Len())
+	for e := listOfStrings.Front(); e != nil; e = e.Next() {
+		s = append(s, e.Value.(string))
+	}
+	return strings.Join(s, ",")
+}